1. Field of the Invention
The present invention relates to a method of providing a coating or film on the surfaces of cutting tools. In particular, the invention relates to a method of coating metal cutting tools by applying to one or more surfaces of the cutting tool a composition containing a liquid metal carboxylate, followed by heat treatment of the cutting tool.
2. Description of Related Art
Cutting tools are used in many different industries, and can generally be thought of as a cutting insert (plate, drill, blade, etc.) fixed on a holder. The cutting inserts are typically made of steel and/or metal-ceramic hard alloys containing tungsten carbide, titanium carbide, tantalum carbide, and other additives. Hard-alloy materials generally contain more that 90% of metal carbides and 3–8% of metal cobalt. One method of making hard-alloy cutting inserts is baking a mixture of metal carbide powders with the metal cobalt powder in a vacuum or reductive atmosphere (argon-hydrogen mixture) at 1300–1500° C.
Cutting inserts are often coated to increase their hardness, to increase their useful life, to increase the time between sharpenings or other maintenance, to prevent fouling of the tool with the material being cut, and to provide other desirable properties, i.e. improvement of the quality of the surfacing process, efficiency increase (cutting speed.) The coatings can be applied to steel cutting inserts and to hard-alloy inserts as well.
Currently, cutting tools are typically coated in one of two ways. One method is chemical vapor deposition (CVD) of oxides, nitrides, and oxynitrides. CVD involves thermal decomposition of volatile metal chlorides (aluminum, titanium, zirconium) on the surface of a cutting tool in a controlled atmosphere containing nitrogen and some oxygen. This process is widely used by major cutting tool manufacturers, such as ISKAR (Israel), Hertel (Germany), and other manufacturers in the U.S., Sweden, Japan.
However, there are several problems associated with the CVD method. First, CVD requires the use of a carefully developed system of safety equipment, as some components used in this process, such as β-diketones and metal-carbonyl complexes, are toxic. Second, the CVD process significantly improves the adhesive properties of the coating, but the coating should be at least 5–15 micrometers thick to provide life extension of the tool. In order to prevent the coating from cracking and peeling it is necessary to match up the respective coefficients of thermal expansion of the cutting tool and the coating. This cannot always be achieved due to differences in these coefficients for the commonly used substrate materials and the common CVD coating materials. Third, the CVD method requires high material and power consumption, increasing the cost of the coated tool. Finally, use of the CVD method results in the production of toxic chloride hydrogen and metal chlorides, whose emission must be controlled from environmental and health standpoints.
The other typical method currently utilized in coating cutting tools is by magnetron spraying of the metal nitrides (using a machine known in Russia as a “Bulat” type machine). During this process a tool and a titanium target plate are placed into a bath filled with nitrogen (pressure about 0.1–1 mm Hg). The bath gets heated up, and the target gets “bombed” by accelerated electrons. Titanium sputters and reacts with nitrogen forming titanium nitride, which settles on the surface of the tool. This process requires control and thorough preparation of the surface of the tool being coated, and it is expensive because of its high material and power consumption.
Other methods for extending the life of tools exist, but cannot be considered as coating processes (for example, nitriding in an ammonia atmosphere, surface modifications in a salt melt). Spraying (plasma, detonation) is primarily used for coating parts for cars and heavy machinery.
Accordingly, there remains a need in the art for a method of coating cutting tools to extend the life of the tool while avoiding the cost and toxic materials handling problems associated with the methods described above.